If you know the colors you prefer and consciously or unconsciously buy plants with flowers in those colors, it usually happens that the color combinations in your garden are harmonious.
For example, I like pinks, blues and purples. This spring, I was happy to see my pale pink ‘New Dawn’ rose blooming next to the purple spires of a sage plant, flanked by pale lavender iris. On a trellis with the rose was a deep purple ‘Jackmanii’ clematis. So there were three different shades of purple harmonizing with the pale pink rose. None of it had been specifically planned but there it was, a splendid blend of colors.
Usually I like to add a lot of white to my garden too, especially in a very hot dry summer, and at dusk, I love what white touches achieve. White candytuft, for example, is a low-growing plant that unifies its neighbors. I also have many shrubs with white flowers: spirea, mock orange, viburnum, itea, rose of Sharon and so on. I enjoy white peonies with purple Siberian iris, and blue baptisia with dark purple salvia ‘May Night’ or blue salvia ‘Butterfly Blue’.
Most of the best combinations in my garden occurred through serendipity, not planning. A useful formula for me is to stick to three favorite colors, plus or minus white.